Treatment Process
Pumping, Screening, Grit
Removal
Sewage enters the treatment plant through multiple force mains from
the three pump stations. All flows come into the plant at a central
head-works facility that houses mechanically cleaned screens. These
screens separate and remove large objects that could harm the
plant's operating equipment. The sewage then flows by gravity to a
five-channel aerated grit removal facility where sand, gravel and
other heavy materials are settled out, cleaned by air agitation,
and fed into storage hoppers for periodic removal and disposal in a
landfill.
Primary
Clarification
Grit removal is followed by primary sedimentation in circular
clarifiers or sedimentation basins, where the heavier inert or
organic solids settle to the bottom and are collected. Sludges from
these basins are pumped to the anaerobic digesters while the
partially clarified effluent overflows the weirs and moves to the
aeration basins.
Aeration Basins
In the aeration basins, the incoming wastewater is completely mixed
with activated sludge that is returned from the bottom of the final
clarifiers. The bacteria present in the mixture remove the organic
impurities from the water and in the process, grow and reproduce.
After several hours in the aeration tanks, the liquid flows into
the final clarifiers where the solids are settled out. A portion of
the solids is returned to the aeration tanks while the excess is
pumped to the anaerobic digesters.
Final Clarifiers and
Disinfection
As the solids settle to the bottom of the clarifiers and are
removed, a clear liquid emerges and gently flows over weirs into
the chlorine contact basins. Here, the clarified effluent is mixed
with chlorine to kill bacteria and other harmful organisms. After
approximately 20 minutes in the presence of the chlorine, the water
flows over exit weirs into a structure before entering pipes that
carry the liquid out of the plant for discharge into either Bird
Creek (Northside) or the Arkansas River (Southside). As the water
flows over the final weirs, it is injected with sulfur dioxide, a
chemical that neutralizes any remaining chlorine.
Sludge Treatment
As wastewater flows through the treatment process and solids are
gradually removed, those solids are pumped through gravity
thickeners on their way to the anaerobic digesters. This process
allows the biosolids to settle once again with additional liquid
being removed to produce a thicker mass of solids. The liquid
effluent from the thickeners flows back into the plant head works
where it mixes with the incoming wastewater and goes through the
treatment process again, getting additional treatment before it is
discharged from the plant. The settled solids in the thickeners are
pumped to the anaerobic digesters. The biosolids remain in the
digesters for a minimum of 20 days while biological activity
continues to occur within the biosolids. Periodically, the digested
solids are withdrawn from the bottom of the digesters and pumped to
lagoons for storage. The biosolids remain in the lagoons for about
one year during which time additional stabilization occurs. The
resulting solids are then pumped into trucks and transported to
area farms and ranches for field application. Each year,
approximately 6,100 dry tons of biosolids are land-applied.
Excess Flow
Treatment
During wet-weather periods, the ground becomes saturated and
rainwater enters the joints of sewer pipes. This can cause above
capacity flows into the treatment plants. The Northside Plant
operates four flow-diversion facilities that are used to store
excess flows during these periods. Collectively they have a storage
capacity of 83.2 million gallons. Following periods of significant
rainfall, when plant flows subside, this stored wastewater is
released back into the sewer system and treated at a managed
rate.
